1. Field of the Invention
This invention relates to an improved process for preparing stain-resistant polyamide fibers, especially carpet fibers, containing cationically-dyeable sulfonate groups along the polyamide polymer chain, wherein the fibers have been dyed with an acid dyestuff.
2. Description of the Related Art
As known in the art, polyamide fibers can be made to resist staining by acid dyes by copolymerizing in the polyamide polymer a small percentage of a cationically dyeable monomer such as an aromatic sulfonate, for example, 1 to 4 weight percent of the sodium salt of 5-sulfoisophthalic acid. In Ucci, U.S. Pat. No. 4,579,762 nylon 6 and nylon 6,6 carpet fibers formed from polymers having aromatic sulfonate units in the polymer chain and having improved stain-resistance to acid dyes are disclosed.
From Windley, U.S. Pat. Nos. 5,164,261 and 5,468,554, it is further known that such cationic-dyeable modified polyamide fibers can be made more stain-resistant upon being dyed with at least 0.0048 weight percent of an acid dyestuff. However, under some circumstances, especially when it is desirable to dye such fibers to only a light shade, it has been found that maximum stain-resistance (i.e., no staining) may not be obtained.
Chao, U.S. Pat. No. 5,030,246 discloses a process for continuously dyeing polyamide fibers which do not contain cationic-dyeable monomeric units in the polymer chain. Rather, the fibers are coated with a stainblocking agent which may contain aromatic sulfonate groups. The fibers can be acid dyed to a deeper shade by adding certain ammonium and metal salts to the dye liquor (typically in an amount from 0.2 to 8% based on the weight of the fiber). Chao discloses that effective salts include salts of lithium,.calcium, and magnesium, as well as certain ammonium, sodium and potassium salts. The use of the salts results in greater uptake of the dyestuff from the dye liquor onto the fibers, and the excess dye not taken up by the fibers is subsequently rinsed off with water.
In Jenkins, U.S. Pat. No. 5,466,527, cationic dyeable nylon fibers containing a sufficient amount of SO.sub.3 H groups or COOH groups within the polymer structure to render the nylon fiber dyeable with cationic dyes are disclosed. A process for improving the stain-resistance, lightfastness, and ozone-resistance of such fibers is disclosed, wherein the fibers are dyed with level acid dyes or premetallized acid dyes at a pH value less than 7.0. In Example 6 (columns 13-14), a carpet made with cationic dyeable nylon fiber is described as being dyed in a dyebath containing level acid dyes or premetallized acid dyes and 2% sodium sulfate (Glauber salt) based on weight of fiber. Tables I and II in Example 6 show that an improvement in dye exhaustion levels is obtained when 2% sodium sulfate is added to the dyebath over dye exhaustion levels obtained from dyebaths which do not contain sodium sulfate. Thus, it is known from Jenkins that sodium sulfate can be added to a dyebath and better exhaustion of the acid dye onto the fiber can be obtained. However, it would be desirable to have a process for acid dyeing cationic-dyeable modified polyamide fibers, wherein the stain-resistance of the fibers can be improved.
The present invention provides such a process. Particularly, it has now been discovered that when certain salts are included in the acid dyebath at certain concentrations, the stain-resistance of the acid dyed fibers is improved. The stain-resistance of the acid dyed fibers is improved without having to increase the dye content on the fiber..